Dumb answer of the day.
These plastic tanks
are vented. You need to twist the cap, open a vent or something.
Is there a black squeeze bulb on the supply line?
Lately what I have seen is a quick disconnect fitting for easy removal
from teh boat, a black squeeze bulb to prime the line, a black cap with the fuel pick up line feeding through it and either a separate vent valve or an inscription that the tank cap need to be clocked to the vent position or some such.
Keep it simple. If you have verified a good fuel supply to the engine, is the engine fuel valve off? Usually you can also open the engine cover, locate the in-line opaque fuel filter and see if the fuel filter is full.
I have seen these with pin leaks
on the inlet side that suck in air.
In this case the symptom your described was manifested.
Squeeze the bulb until it got hard, start the engine, the fuel filter would suck in air, the engine would almost die, squeeze the now soft bulb and it would fill the filter and harden up again.